(a) Field
The subject matter disclosed generally relates to filtering apparatus and methods of using the same. More particularly, the subject matter relates to media bed filters for filtering fine particles from a raw liquid flow.
(b) Related Prior Art
Media bed filters work by providing the solid particles with many opportunities to be captured on the surface and within a filtering media bed. As fluid is evenly distributed at the top of the filter, it gently flows through the porous sand (i.e., filtering media) along a tortuous route, the particles come close and in contact with the media bed. They can be captured by one of several mechanisms such as, direct collision, Van der Waals or London force attraction, surface charge attraction, diffusion, and the like.
In addition, solid particles can be prevented from being captured by surface charge repulsion if the surface charge of the filtering media is of the same sign (i.e., positive or negative) as that of the particulate solid. Furthermore, it is possible to dislodge captured solid particles although they may be re-captured at a greater depth within the media bed.
Filtering media beds can be operated either with upward flowing fluids or downward flowing fluids the latter being much more usual. For downward flowing filtering media beds, the fluid can flow under pressure or by gravity alone. Pressure media bed filters tend to be used in industrial applications. Gravity fed units are used in water purification especially in large application such as drinking water.
Overall, there are several categories of filtering media beds such as, without limitation, gravity media bed filters, pressure media bed filters, upflow media bed filters, slow media bed filters, multimedia bed filters and the like.
All of these apparatus and methods are used extensively in the water industry throughout the world.
For example, water from cooling tower attracts and absorbs most dirt and airborne on a continuous basis. The majority of suspended solids in circulating cooling water loops are from about 0-5 micron in size, mainly because of chemical dispersing agents that are designed to limit circulating (i.e., dust and minerals kept in suspension by dispersing chemical agents) dirt from agglomerating on heat exchange surfaces. Dirt does negatively affect heat exchange surfaces and cooling tower fill efficiency. Traditional filters, strainers and separators will not remove significantly these very fine contaminants before they settle out in low flow areas, clog strainers, nozzles, and bio-fouled heat exchangers. Usually, most media bed filters of this kind are not able to significantly retain suspended solid of less than 5 microns in size. There is therefore a need to provide a media bed filter designed to provide an improved filtration for fine particles down to 0.5 microns. For example, a traditional multi-layers media bed filter having 3 layers including garnet is able to filter particles only down to 10 or 20 microns.
For example and referring now to Prior Art FIGS. 1A, 1B, 1C, 1D and 1E, there are shown traditional sand filters. These traditional sand filters offer a plurality of disadvantages. One of them is that, a slope is created by the raw liquid fluid entering the tank. The prior art configuration will allow the raw liquid flow to dig at one place only on the media bed. Thus, according to the traditional media bed filter, only a portion of the media bed is utilized as the filtering surface. One of the other disadvantages is that traditional sand filters cannot be used at greater flow rates. When using traditional sand filters, water needs to enter the tank at a substantially small velocity and cannot include many flow rate variations. Additionally, such configurations proposed by traditional media bed filters may lead the particles to form a cake layer on the top portion of the media bed and may also block the media bed filter. Thus, the maintenance of such media bed filters needs to be made on a regular basis for reducing formation of cakes with the media bed. For example, is FIG. 1A, the raw liquid flow which enters the tank follows a laminar flow (i.e., without or with reduced turbulence areas).
Many filters are already known in many applications, such as, without limitation, chilled and hot water loops, condensate return, cooling tower make up, iron removal, ion exchange resin pre-filtration, membrane pre-filtration, potable water and beverage filtration, process rinse water, process water intake, water reuse, welder water loops and the like.
Moreover, traditional filters will require coagulants or polymers to improve their efficiency towards smaller particles. Existing vortex filters have the disadvantage of having poor backwash efficiency, resulting in higher water consumption, wastewater and important energy costs.
Traditional vortex filters do not allow good backwash efficiency and are prompt to short-circuiting even when clean. In fact, the single injector located at a significant distance from the apex of the tank creates a significant distortion of the fine sand surface (FIG. 1B) (i.e., also called microsand or ultrafine sand) with one side of the media bed being significantly deeper than its opposite side creating a significant slope in the filtering media of about 30 to about 40°. This slope creates a distortion in the hydraulic distribution of the fluid at the surface and in the depth of the media bed. This phenomenon does not allow the known vortex filter to use efficiently the filtration surface area. This is especially true for filters of larger surface such as 30 inches of diameter and above. As for the backwash process, the typical single injector, located at a significant distance from the apex of the tank, does not allow for a good capture of the particles (or fine particles) to be removed as this design does not allow for a plug flow removal process. It is to be noted that the configuration as shown in FIG. 1B would not result in a good hydraulic flow. The media bed, and more particularly the filtering media is significantly deformed by the water flow which enters the tank at a significant distance from the apex of the tank.
Furthermore, open-tank media bed filters include a raw liquid flow inlet which is configured so to lead the water gently above the filtering media so that the particles flow gently within the filtering media, and the filtering media surface is not in motion nor disturbed.
There is therefore a need for improved media bed filters for filtering and backwashing fine particles from a raw liquid flow and for methods of using the same.